In a principal aspect, the present invention relates to a high efficiency, multistage liquid heating apparatus utilizing combustible fuel as a heat source.
In a liquid heating apparatus, such as a water heater, the liquid to be heated typically is introduced into the lower portion of a tank. As the liquid is heated, it becomes less dense, thereby rising to the top of the tank where it is drawn off.
A heat source for such an apparatus may be a combustible fuel such as natural gas that is burned in a combustion chamber located beneath the tank. The hot gases produced by combustion rise through flues that pass through the tank and serve as a heat exchanger to conduct the heat from the rising hot gas to the liquid inside the tank. Since heat is removed from the hot gas as it moves upward, the temperature of the gas within the flues decreases as it rises. Thus, the temperature difference, and consequently the rate of heat transfer between the hot gas and the liquid to be heated is greatest at the bottom of the tank, and least at the top of the tank.
Historically, multiple staging of liquid apparatae that utilize combustible fuel as a heat source has been commonly employed to increase the theoretical efficiency of heat transfer. In a multiple stage liquid heating apparatus, the coldest liquid enters the uppermost tank and is transferred through successively lower tanks in a serial fashion. This raises the theoretical efficiency of the apparatus because a large temperature difference between the liquid to be heated and the hot gas is maintained throughout the length of the liquid heating apparatus.
A problem associated with a multistage gas fired water heater is that the water vapor formed as a product of combustion condenses in the flues of the upper stages. This is because the cooler rising flue gas contacts the tanks containing the cold water to be heated. The condensate forms water droplets which trickle down the inner walls of the flues. The water eventually trickles down the inner walls of the lower stages. Such condensation not only encourages corrosion of the flue and tank and interrupts the upward movement of hot gases, it also decreases the heat transfer coefficient of the apparatus and thereby reduces its efficiency. The thin film of water that forms on the surface of the flues is responsible for lowering the real heat transfer efficiency of the apparatus.
Another problem associated with multistage liquid apparatae is that the presence of complex flue assemblies interferes with the ability to clean the flues. Since the flues communicate the by-products of combustion from the burner to the exhaust manifold, carbonaceous build-up in the flues is inevitable. Failure to remove this build-up will result in a lower heat transfer efficiency for the apparatus.
The present invention constitutes a multistage liquid heating apparatus that seeks to overcome the problems resulting from water vapor condensation in the upper flues while at the same time providing a simple, easily constructed design that is more easily cleaned.